1. Field of The Invention
This invention relates to diaphragm correcting means of an objective lens mounted on a single lens reflex camera of the TTL full aperture exposure measurement type. The expression--full aperture exposure measurement--refers to a system for measuring the light transmitted through the objective lens when the lens aperture is fully opened.
2. Description of the Prior Art
In a single lens reflex camera of the TTL full aperture exposure measurement type wherein, for example, shutter speed priority automatic exposure control is effected, the preset diaphragm ring of the lens is set to a position in which a minimum aperture may be obtained to enable the stop-down of the diaphragm from its maximum aperture to its minimum aperture. Then, it is required to stop down the diaphragm from its maximum aperture to a proper aperture value determined by a preset shutter speed, film sensitivity, brightness of an object, etc. The metering output of a metering circuit included in the automatic exposure control circuit of TTL full aperture exposure measurement type of such a single lens reflex camera is substantially proportional to the area of the aperture formed by the diaphragm, but in lenses having a small open F-value such as F/1.2 or F/1.4, the aforementioned proportional relationship tends to become imprecise in the vicinity of the maximum aperture to thereby reduce the metering output. As the reason therefore, mention may be made of a reduction in the quantity of marginal light of the lens, and the characteristic of the construction of the metering optical system; for example, the fact that the greater the maximum aperture of the objective lens, the more difficult it is for a condenser lens of the metering optical system to condense the marginal light of the lens. In the case of the TTL full aperture exposure measurement, such imprecision of the mentioned proportional relationship appears directly as an exposure error. To correct such exposure error attributable to the open F-value of the lens, German Patent No. 1,258,727 discloses a technique in which a correcting cam is operatively associated with the preset diaphragm ring of the lens and an exposure meter needle is corrected and displaced by a lever following the correcting cam. This technique is effective in the case of an aperture priority system in which the photographer presets the aperture value and determines the diaphragm aperture by an aperture ring interlock member, but is not effective in the case of a shutter speed priority system in which a lever on the camera housing side determines the area of the aperture opening through a diaphragm driving lever on the lens side.